Interest Days Calculator (Day-Count Conventions)
Two parties can agree the same principal and rate yet compute different interest because they count days differently. Banks, bond markets and foreign lenders use distinct day-count conventions. Enter principal, annual rate and the start/end dates and this tool shows the number of interest days and the interest under each convention, so you can check a bank's interest certificate, a loan sanction letter, an inter-corporate deposit, or statutory interest.
The four conventions
- Actual/365 (Fixed) — actual calendar days ÷ 365. Standard for Indian bank loans, fixed deposits, savings interest and statutory interest (income-tax, GST, TDS interest is "per day" on a 365 basis).
- Actual/360 — actual days ÷ 360. Common in money markets, ECBs and USD/EUR loans; yields about 1.39% more interest than Actual/365 for the same quoted rate.
- 30/360 (Bond basis) — every month treated as 30 days, year as 360. Used for corporate bonds, debentures and many lease schedules; 31st dates are adjusted to 30.
- Actual/Actual (ISDA) — actual days ÷ 365, or ÷ 366 for the portion falling in a leap year. Used for government securities and swaps.
Formula
Interest = Principal × Rate × Year Fraction, where Year Fraction = Days ÷ Basis. With compounding selected, Amount = P × (1 + R/n)n × fraction − P (quarterly compounding mirrors bank FD/SB practice).
Worked example
₹5,00,000 at 9% p.a. from 01-01-2026 to 01-04-2026 (90 days, 2026 is not a leap year):
- Actual/365: 90 ÷ 365 × 9% × 5,00,000 = ₹11,095.89
- Actual/360: 90 ÷ 360 → ₹11,250.00
- 30/360: 90 ÷ 360 → ₹11,250.00
- Actual/Actual: same as Actual/365 here → ₹11,095.89
Inclusive counting
By default the end date is excluded (interest accrues from the start day up to, but not including, repayment day). Tick the inclusive option if the contract says "both days inclusive". A daily-interest figure is also shown for quick per-day accruals in provisioning entries.
Frequently Asked Questions
Which convention do Indian banks use?
Actual/365 for most rupee loans, FDs and savings accounts; some lease and bond products use 30/360.
Why does Actual/360 give more interest?
Dividing by 360 instead of 365 inflates the year fraction by 365/360 ≈ 1.0139, so the lender earns ~1.4% more.
Is the end date counted?
No by default (standard practice). Use the "include both dates" option when the agreement says so.
How is a leap year handled?
Actual/Actual uses 366 for days falling in a leap year; Actual/365 always divides by 365.
Disclaimer: This tool is for general information and estimation only. Tax rates and rules change — verify with official sources or a professional before acting. Read full disclaimer.